Densitometer

ABSTRACT

A densitometer which utilizes a thin vane vibrated by a magnetostrictive tube. A piezoelectric crystal pickup feeds an amplifier and tracking filter to energize a coil around the magnetostrictive tube. The device is, in effect, an electromagnetic oscillator in that the coil is driven in phase with the detected signal. Vibration thus increases in amplitude until limited electrically. A linearization circuit provides an output D.C. .voltage directly proportional to fluid density, gas, or liquid, which may be impressed upon a conventional D.C. voltmeter calibrated linearly in density. A great many features, both in the mechanical structure and in the electronics, make it possible to easily calibrate the instrument to read, for example, to within an accuracy tolerance of + OR - 0.01 percent over a fluid density range of, for example, from about 0.08 pounds per cubic foot to 80.0 pounds per cubic foot. Another outstanding advantage of the invention relates to a probe type construction which may be used in a pipeline of any size.

United States Patent 1 1 Jan. 30, 1973 Miller et al.

Division of Ser. No. 65,371, Aug. 20, 1970, Pat. No. 3,677,067.

US. Cl. .....73/l R, 73/32 Int. Cl ..G0ln 9/00 Field of Search .1 ..73/1R, 32 A References Cited UNITED STATES PATENTS 3/1967 Banks ..73/1R11/1965 Sipin ..73/32A Primary Exantiner--S. Clement Swisher Attorney-C.Cornell Remsen, Jr. et al.

[57] ABSTRACT A densitometer which utilizes a thin vane vibrated by amagnetostrictive tube. A piezoelectric crystal pickup feeds an amplifierand tracking filter to energize a coil around the magnetostrictive tube.The device is, in effect, an electromagnetic oscillator in that the coilis driven in phase with the detected signal. Vibration thus increases inamplitude until limited electrically. A linearization circuit providesan output DC .voltage directly proportional to fluid density, gas, orliquid, which may be impressed upon a conventional D.C. voltmetercalibrated linearly in density. A great many features, both in themechanical structure and in the electronics, make it possible to easilycalibrate the instrument to read, for example, to within an accuracytolerance of t 0.01 percent over a fluid density range of, for example,from about 0.08 pounds per cubic foot to 80.0 pounds per cubic foot.Another outstanding advantage of the invention relates to a probe typeconstruction which may be used in a pipeline of any size.

1 Claim, 11 Drawing Figures DENSITOMETER This is a division of copendingapplication Ser. No. 65,371, filed Aug. 20, 1970, now U.S.- Pat. No.3,677,067. The benefit of the filing date of said copending applicationis, therefore, hereby claimed for this application.

BACKGROUND OF THE INVENTION This invention relates to instruments forproducing output signals as a function of the density of a fluid, andmore particularly, to a vibration densitometer.

In the prior art, densitometers have been developed in which structureshave been submerged in a fluid and vibrated. The frequency of vibrationof the structure was then a function of the density of the fluid. In allsuch prior art densitometers, either the entire flow of a fluid in apipeline was directed through the densitometer, or a bypass wasconstructed to divert a portion of the fluid through the densitometer.Both of these arrangements of the prior art have had several seriousdisadvantages. In the first place, those instruments taking the entirepipeline flow could not be used where the pipeline was even moderatelylarge. This was true because it was impractical to make a densitometerfairly large. It was also impractical to make densitometers in a greatmany sizes to fit pipelines of all sizes.

In the prior art instruments utilizing the bypass, pressuredifferentials and changes due to the bypass connection have caused largeerrors in density measurement.

In addition to the foregoing, vibration densitometers of the prior arthave required the use of a great many expensive and complicatedcomponent parts.

SUMMARY OF THE INVENTION In accordance with the device of the presentinvention, the above-described and other disadvantages of the prior 'artare overcome by providing a vibration densitometer with a probe forinsertion into a fluid. In accordance with the present invention, theprobe may thus be inserted into the fluid in any manner. However, theoutput of the densitometer of the present invention may often be used inthe measurement and indication of the density of a gas or liquid in apipeline. Moreover, the output of the densitometer of the presentinvention may also be used in combination with a volume rate of flowsignal to produce an indication of the rate of mass flow or the totalmass flow through a pipeline.

Since the densitometer of the present invention has an immersible probe,the probe may thus be simply mounted through a suitable opening in thewall of a pipeline. The inside diameter of the pipeline thus does notrestrict the use of the device of the invention. In other words, it maybe used in a pipeline of any size. Further, the device of the inventionis extremely accurate and, in addition, is not subjected to the pressuredifferentials and changes attendant upon the use of the prior artbypass. For example, the densitometer of the present invention may beeasily calibrated, for example, to within an accuracy tolerance of 0.01percent over an extremely wide range of densities. That is, the widerange is considerably larger than that of any range of any known priorart densitometer. For example, the densitometer of the present inventionmay be calibrated to within the said 10.01 percent accuracy over a rangefrom about 0.08 pounds per cubic foot to about 80.0 pounds per cubicfoot.

Notwithstanding the foregoing outstanding features of the presentinvention, the present invention requires only the use of relatively fewinexpensive and uncomplicated component parts. For example, thevibrating structure used in accordance with the present invention maysimply be a vane. For example, the vane may be a thin membrane havingalso parallel flat surfaces. The vane may be rectangular and haveexternal surfaces defining a right parallelopiped. Preferably, the vaneis made of any conventional metal which will not erode with use. Forexample, the vane may be made of stainless steel.

In addition to the foregoing, the densitometer of the present inventionhas a great many other advantages, many of which are also outstanding.Some of these advantages are outlined hereinafter. For example, thereare many mechanical arrangements in the probe which make the use ofcomplex component'parts unnecessary. The use of a piezoelectric crystalpickup also makes it possible to reduce the size of the probeconsiderably.

Another advantage of the invention relating to approved efficiency andaccuracy relates to a structure for clamping the edges of the vanes witha pressure of, for example, 20,000 pounds per square inch.

Many component parts of the densitometer of the present invention, andcombinations thereof, perform two or more functions simultaneously. Oneexample of such a combination is the use of an outer magnetic tube andan inner magnetostrictive tube which form, more or less, the body of theshank of the probe. The use of magnetic materials in both inner andouter tubes make it possible to shield electrical leads to a driver coilbetween the tubes and leads from the crystal through the center of theinner tube. That is, the leads from the driver coil are maintainedbetween tubes, and the leads from the crystal are positioned inside theinner tube. This prevents currents from being induced in the crystalcircuit by the current in the driver coil leads. The signal on thecrystal leads is amplified before all four leads are unshielded.

A further advantage of the present invention is the construction of theinner and outer tubes to form a substantially completely closed magneticcircuit around the driver coil which is located in a space between thetubes. This makes the driver coil magnetic coupling with the inner tubea maximum. The magnetostrictive inner tube, which is employed to vibratethe vane, is thus driven with a maximum efficiency.

Another advantage of the present invention resides in the use of anouter tube construction which holds at least a portion of the inner tubein axial compression against a driver member at all times. That is, theinner tube is in axial compression both when the driver coil isenergized and when the driver coil is deenergized. The degree of axialcompression thus changes when an alternating current is passed throughthe driver coil. The efficiency of vibration transmitted from the innertube is thus increased. There-is no wasted motion, and there is noenergy loss due to hammer. That is, the inner tube does not hammer anystructure.

Another advantage of the invention resides in the use .of a resilientmounting for connection between the probe and, for example, a pipeline.

Another feature of the invention resides in the use of a resilient sealfor the probe which performs at leasttwo functions. The seal firstperforms the resilient mounting function previously described, and italso seals off the fluid and prevents a leak thereof from between theprobe of the opening in the pipeline in which the probe is inserted. Theresilient mount of the probe substantially increases the efficiency andaccuracy of the instrument.

Efficiency and accuracy of the densitometer of the present invention isalso increased by the use of a resilient mount for anelectricalconnector otherwise substantially fixed relative to the probe.

Another feature of the invention resides in means for impressing asignal upon the driver coil responsive to the output signal of thecrystal. The driver coil is supplied with a signal in phase with theoutput of the crystal. The densitometer of the present invention thusacts, in part, as a electromagnetic oscillator. If desired, theoscillator can be prevented from running away by limiting the outputsignal magnitude of the means employed to impress a signal on the drivercoil.

The means for controlling the driver coil may include a differentiatorand a tracking filter. It is an advantage of the invention that thedifferentiator can attenuate the lower frequencies while applying aphase shift at 90 at all frequencies. Thus, the tracking filter likewisemay be connected to provide a phase shift over 90 in the same direction,and the driver coil signal may be set in phase merely by connecting thedriver coil leads with the proper polarity.

Another feature of the invention resides in the use of a highly accurateanalog conversion apparatus for taking a signal of onefrequency andconverting it to another function of frequency. By the use of thisdevice, it is possible to obtain a D.C. output voltage directlyproportional to the density of the fluid being metered. Thus, a D.C.voltmeter calibrated linearly in density may be employed as anindicator.

Another feature of the present invention resides in the use of anextremely simple method of calibrating the densitometer to within thesaid 10.0l percent accuracy over the said wide range.'

For purpose of definition herein and in the claims, the phrase firstresonant frequency hereby is defined to mean the lowest frequency atwhich the said electromagnetic oscillator will oscillate. Note will betaken that depending upon the location and band width of the trackingfilter passband, the said electromagnetic oscillator may oscillate atany one of several resonant frequencies. It is also to be pointed outthat whichever resonant frequency is selected, the resonant frequencychanges with density and it is this change in resonant frequency thatproduced an indication of the density of a fluid.

Another feature of the invention resides in the said resilient probemounting. Thisresilient probe mounting keeps the said electromagneticoscillator from jumping from one resonant frequency to another withoutany outside stimulus.

As will be explained, the lowest resonant frequency is. preferredbecause, in general, the largest signal-tonoise ratio maybe obtained forthis condition.

The word densitometer is hereby defined for use herein and in. theclaims to include an instrument without, as well as with, a density orother indicator. For example, the device of the present invention canproduce an output signal which is a D.C. voltage directly'proportionalto density. Thus, if the invention were combined into a mass rate offlow meter, the indicator would indicate the mass rate of flow, and notdensity. It would thus be unnecessary to have an indicator to indicatedensity. The said D.C. analog voltage would then only be used in amultiplier to derive a signal directly proportional to, for example,theproduct of the density and volume flow rate analogs. The outputsignal magnitude would then be directly proportional to mass rate offlow. The magnitude of a mass rate of flow analog could be displayed onan indicator. Alternatively, a total mass flow indicator could be usedby integrating the multiplier output. A density or mass rate of flow orother analog could be used in controlling a process. In this case, noindicator whatsoever would be needed.

The word differentiator, as used herein, includes an amplifier. Thisdifferentiator is entirely conventional, but is not identical to otherconventional differentiators of the prior art. Thus, the worddifferentiator is defined for use herein and in the claims to includethe differentiator shown in the drawings and described herein and anyequivalent thereof.

From the foregoing, it will be appreciated that the densitometer of thepresent invention may be used in a pipe or another container of a fluidtight construction.

However, the pipe or other container need not be fluid tight. A simplefluid receptacle will do as well. Thus, the probe of the invention maysimply be submerged in a fluid. Further, it is a feature of theinvention that the probe may be inserted either into a gas or a liquid,or both in succession, and the density of the gas or the density of theliquid or the densities of both may be determined even without a changein calibration.

The above-described and other advantages of the present invention willbe better understood from the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings which are to beregarded as merely illustrative:

FIG. 1 is a perspective view of a densitometer probe constructed inaccordance with the present invention;

FIG. 2 is a sectional view of the probe taken on the line 2-2 shown inFIG. 1;

FIG. 3 is a perspective view of a group of component parts of the probeshown in FIG. 1;

FIG. 4 is a transverse sectional view of the assembly taken on the line4-'-4 shown in FIG. 3',

FIG. 5 is an enlarged longitudinal sectional view of a portion of theprobe shown in FIG. 1;

FIG. 6 is a longitudinal sectional view of a portion of mounting meansfor an electrical connector otherwise substantially fixed relative tothe probe taken on the line 6-6 shown in FIG. 2; I

,FIG. 7 is a block diagram of a densitometer constructed in accordancewith the present invention;

" FIG. 8 is a schematic diagram of a current-to-voltage converter shownin FIG. 7;

'- FIG. 9 is a schematic diagram of a differentiator shown in FIG. 7;

FIG. is a schematic diagram of two of the blocks shown in FIG. 7, twoother of the blocks also being shown in relation thereto; and

FIG. 11 is a schematic diagram of a linearization circuit shown in FIG.7.

DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 1, the probe of thepresent invention is indicated at 10 having a shank 11, a housing 12 atits upper end, a tubular assembly 13 at its lower end, and an electricalconnector assembly 14 at the upper end of housing 12 fixed thereto bybolts 15. Annular fittings 16 and 17 extend around shank 11 for mountingprobe 10 in a hollow cylindrical extension 18 of a pipeline 19, as shownin FIG. 2.

As shown in FIGS. 1 and 2, a stainless steel vane 20 is mounted inassembly 13 in a position perpendicular to the axis of a hollowcylindrical magnetostrictive inner tube 21. Vane 20, if desired, may bealso mounted in a symmetrical position with respect to the axis of anouter sleeve 22 which houses it.

Vane 20 may be a rectangular plate having flat and parallel upper andlower surfaces as shown in FIG. 2, and may otherwise have mutuallynormal surfaces forming a .right parallelopiped.

Shank 11 not only includes inner tube 21, but an outer magnetic tube 23.A driver coil or solenoid winding 24 wound on a nylon bobbin 25 is pressfit onto the external surface of inner tube 21 and located in a spacebetween the tubes 21 and 23 toward the lower end of shank 11. Coil 24 isthus maintained in a substantially fixed position on inner tube 21,although the same is not necessarily critical to the operation of thedevice of the present invention.

Vane 20 is supported between two half cylinders 26 and 27 as shown inFIGS. 2 and 3. According to the invention, the longitudinal edges ofvane 20 are pressed together between half cylinders 26 and 27 with apressure of, for example, 20,000 pounds per square inch because theassembly shown in FIG. 3 is inserted in sleeve 22 with an interferencefit, sleeve 22 being heated prior to the said insertion.

I-Ialf cylinder 26 has four projections 28, and half cylinder 27 hasfour projections 29. Projections 28 and 29 serve to prevent longitudinalmovement of vane 20 between half cylinder 26 and half cylinder 27although the same is not likely due to the clamping pressure on vane 20between half cylinder 26 and half cylinder 27.

Half cylinders 26 and 27, and vane 20 may be machined to have a flat orrecess to receive a piezoelectric crystal 30. Crystal 30 has electricalleads 31 and 32 which extend around half cylinders 26 and 27 in grooves33 and 34, respectively, to a point where they enter the hollow interiorof inner tube 21. This entry is made at the lower end of inner tube 21,as shown in FIG. 2.

As shown in FIG. 3, projections 28 and 29 may have a slight separationat 35 to insure that the pressure contact of half cylinders 26 and 27 onvane 20 is quite high due to the said interference fit.

As shown in FIG. 2, a boss 36 is welded at 37 to sleeve 13 in a fluidtight manner. Although the device of the present invention need notalways be fluid tight throughout, a glass-to-metal seal or other sealmay be provided inside inner tube 21 for leads 31 and 32. Before thesaid interference fit is provided, if desired, crystals 30, and thoseportions of leads 31 and 32 in grooves 33 and 34, respectively, may bepotted with an epoxy. Further, after the interference fit has beeneffected, the entire unit when completely assembled may be treatedfurther by applying a bonding agent around all of the structures insidesleeve 22. Any conventional bonding process may be employed including,but not limited to, the application of a bonding agent sold under thename of Locktile.

As stated previously, boss 36 may be welded to sleeve 22 at 37 in afluid tight manner. Further, outer tube 23 may be threaded onto boss 36and welded thereto at 38 in a fluid tight manner. For all practicalpurposes, boss 36 may thus be considered an integral part of outer tube23. Boss 36, for example, is also made of a magnetic material. All ofthe magnetic materials" referred to herein may be any magnetic materialincluding, but not limited to, stainless steel. However, inner tube 21,although being magnetic, must also be magnetostrictive. Notwithstandingthis limitation, it is to be noted that inner tube 21 is employed toproduce vibration, and if one feature of the present invention is usedwithout another, the use of a magnetostrictive or magnetic material maynot be required, and the invention still practiced.

Inner tube 21 has an annular projection 39 with a shoulder 40. Outertube 23 has a lower bore 41 separated from a smaller upper counter bore42 by an annular shoulder 43. Shoulder 40 and 43 abut. From shoulder 40to the lower end of inner tube 21, inner tube 21 is always in axialcompression. That is, inner tube 21 is in compression when coil 24 isenergized, but inner tube 21 is also in compression when coil 24 isdeenergized. Coil 24 is energized with an alternating current which thusmerely changes the degree of compression of inner tube 21.

Projection 39 has a hole 44 through which the electrical leads of coil24 can pass from the location of coil 24 upwardly between tubes 21 and23.

The manner in which probe 10 is mounted in pipeline 19 is betterillustrated in FIG. 5. In FIG. 5, note will be taken that outer tube 23has an outwardly extending radial projection 45 on each side of whichrubber O-rings 46 and 47 are compressed by fittings 16 and 17. Fitting17 is threaded into extension 18 and sealed thereto by a conventionalsealing compound 48 shown in FIG. 2. In FIG. 5, note will be taken thatfitting 16 is threaded inside fitting 17 at 49. The amount Orings 46 and47 are compressed is, therefore, determined by the position of fitting16. That is, fitting 16 is turned, for example, by a wrench, until thedesired O-ring compression is reached.

From the construction illustrated in FIG. 5, note will be taken thatonly O-rings 46 and 47 contact outer tube 23, and that, therefore, shank11 is never touched by either fitting 16 or fitting 17.

It is an advantage of the present invention that the construction ofprobe 10 is such that the leads from coil 24 are kept magneticallyseparate from the leads from crystal 30. This is true through a portionof housing 12 as will be described. Housing 12 has a fitting 50 threadedonto outer tube 23. A cylinder 51 is threaded to fitting 50. A washer 52is press fit and thereby fixed in fitting 50 and inner tube 21. Innertube 21 has an upper end which may be fixed relative to or slidable inwasher 52, as desired. However, preferably the external surface of innertube 21 at its upper end fits contiguous or in contact with the surfaceof washer 52 defining the hole therethrough. A shield 53 made of amagnetic material may be fixed around fitting 50 by one or two or morescrews 54. Outer tube 23 has a radial hole 55 therethrough through whichthe leads from coil 24 pass. Fitting 50 has a hole 56 therethrough inalignment with hole 55 through which the leads from coil 24 pass. Fromthe outward radial extremity of hole 56, the coil leads indicated at 57and 58 pass upwardly between cylinders 51 and shield 53 and areconnected to pins 59 and 60 of the electrical connector 14. Electricalconnector 14 may be a conventional five pin connector.

As stated previously, the leads 31 and 32 from crystal 30extend'upwardly through the interior of inner tube 21. At the upper endof inner tube 21, as shown in FIG. 2, leads 31 and 32 are connected tothe input of differential amplifier 61. Leads 31 and 32 thus extendoutwardly through the upper opening in inner tube 21'.

Differential amplifier 61 may be entirely conventional, and mounted on aconventional card, if desired. Amplifier 61 may be supported insideshield 53 by any conventional means, if'desired, or simply supported bythe strength of leads 31 and 32, and output leads 62 and 63 which areconnected to pins 64 and 65 of connector 14, respectively. A lead 66provides a ground connection from shield 53 to the fifth pin 67 ofconnector 14.

The manner in which connector 14 is mounted on cylinder 51 is shown inFIG. 6. Only one bolt is shown in FIG. 6 since all bolts 15 aresimilarly situated. In FIG. 6, bolt 15 is shown having a head 68, awasher 69 under head 68, an O-ring 70 under washer 69, and a shank 71threaded into cylinder 51. A second O-ring 72 also extends around screwshank 71. O-ring 70 fits between the lower surface of washer 69 and acounter sunk frustoconical hole 73 in connector 14. O-ring 72 fitsbetween the upper surface of cylinder 51 and another counter sunkfrusto-conical hole 74 in connector l4. Holes 73 and 74 are connected bya bore 75. From FIG. 6, it will be noted that all the structures showntherein may vibrate, but that the amount of vibration transmitted toconnector 14 may be quite small.

One embodiment of the densitometer of the present invention isillustrated in FIG. 7. Probe 10 is again so indicated as housing drivercoil 24, crystal 30, and differential amplifier 61. Crystal 30 islabeled detector in FIG. 7.

A current-to-voltage converter 76 is connected from amplifier 61.Converter 76 is shown in FIG. 8, and may be entirely conventional. InFIG. 8, an amplifier is indicated at 77 having a feedback resistor 78connected from its output at 79 to its input at 80, amplifier 77 havinga ground connection at 81.

In FIG. 7, a differentiator 82 is connected from converter 76 to asquarer 83. Differentiator 82 may likewise be entirely conventional, asshown in FIG. 9.

In FIG. 9, an amplifier 84 has a feedback resistor 85 connected from anoutput 86 to an input 87. The input to the differentiator 82 is thensupplied through a capacitor 88 connected to the amplifier input 87.Amplifier 84 is also supplied with a ground connection 89.

The input to differentiator 82 is mainly a sine wave voltage having afrequency which is equal to the resonant frequency detected by crystal30. As is conventional, differentiator 82 then produces positive andnegative output pulses alternately at the O crossover points of the sinewave. The output of differentiator 82, i.e., the pulses, are thenconverted to a square wave by squarer 83. Since the units of time arenot equal to the units of potential, the phrase square wave is,therefore, defined for use herein and in the claims to means a voltagewave which abruptly arises to a maximum value and stays constant overhalf the period thereof, and then abruptly drops, for example, with analmost infinite slope again to its minimum value. The square wave thenremains at its minimum value for half of its period. Thus, a square wavemay have any maximum amplitude and any minimum amplitude without regardto its period or frequency.

An amplitude control 90, a tracking filter 91, and a power amplifier 92are successively connected from squarer 83 to driver coil 84. A phasecomparator 93 receives one input from the output of control 90, anotherinput from the output of filter 91, and supplies an input to a filterfrequency control 94. The output of the control 94 is employed to varyelectrically the frequency location of the passband of filter 91 towhere the signal having the fundamental frequency of the square waveoutput of control 90 to pass through filter 91 to-its output with theleast attenuation.

Amplitude control 90 may simply be a voltage divider to reduce theamplitude of the output signal of squarer 83 to a desired value. Notethat if all of the blocks of the system of FIG. 7 previously describedoperate as an electromagnetic oscillator, the oscillation amplitude mayincrease to infinity at which or before which some of the componentparts may fail. Thus, to put a finite limit on the amount of feedback todriver coil 24, control 90 is provided.

Power amplifier 92 produces an alternating output voltage whose averageamplitude is somewhat above or below zero. That is, it has aconventional D.C. bias as explained in many publications including, butnot limited to Magnetostriction Transducers, published by TheInternational Nickel Company, Inc., 67 Wall Street, New York, New York10005. See also, for example, the bibliography of this one publication.The D.C. bias is employed to keep the current flow through driver coil24 in one direction only and to keep the frequency of the output voltageof crystal 30 equal to that of the input voltage to the driver coil 24.

Phase comparator 93 is entirely conventional. Control 94 and filter 91are shown in FIG. 10. Filter 91 includes an amplifier 95, a resistor 96connected from the output to the input of amplifier 95, a capacitor 97connected from junction 98 to the output of the amplifier 95, acapacitor 99 connected from the junction 98 to the input of amplifier95, and a resistor 100 connected from the output of control 90 tojunction 98.

Control 94 includes an amplifier 101 connected from the 'output of phasecomparator 93 through a resistor 102. A resistor 103 is connected fromthe output to the input of amplifier 101. Similarly, a capacitor 104 isconnected from the output to the input of amplifier 101. The output ofamplifier 101 is connected to the gate 105 of a field effect transistor106. The source 107 of transistor 106 is connected to junction 98. Thedrain 108 of transistor 106 is connected to ground.

The output of filter 91 is taken at junction 88 and applied both toamplifier 92 and comparator 93.

In FIG. 7, the output of filter 91 is impressed upon a linearizationcircuit 109. The output of circuit 109 is impressed upon an indicator110, which may be a voltmeter as shown in FIG. 11. Voltmeter 110 may bea D.C. voltmeter linearly calibrated in density.

If desired, so that phase comparator 93 may receive a stronger inputsignal, the output of squarer 83 may be connected to comparator 93, andthe input thereto from the output of amplitude control 90 may beomitted. Similarly, the connection between the output of filter 91 andcomparator 93 may be omitted and squarer may be connected from theoutput of filter 91 to the right-hand input of comparator 93, as shownin FIG. 7. If the squarer is inserted the linearization'circuit 109 mayalso receive its output.

Linearization circuit 109 is shown in greater detail in FIG. 11including divide by two circuits 111 and 112, both of which may beidentical. Each divide by two circuit may be entirely conventional. Forexample, divide by two circuit 111 may be simply a binary digitalcounter adapted for automatic reset on a predetermined count. The outputof this counter would then be taken from the last stage thereof.

In general, circuit 111 will produce a square wave output from apositive maximum to ground. The square wave would thus never drop belowground. A relatively large capacitor 113 centers the square wave aboutground so that the square wave reaches an approximately maximum value of+E, and a minimum value of E,. As the frequency of the input signal ofcircuit 111 is fl,, the frequency of the output signal of circuit 111 isf. Thus, f K f. K, may be any number larger than zero. However, in thespecific case of circuit 111, K 2.

A resistor 114 is connected from capacitor 113 to the input of anamplifier 115. A capacitor 116 is connected from the output to the inputof amplifier 115. The same is true of a switch 117. Switches are alsoprovided at 118, 119 and 120. All of the switches 117, 118, 119 and 120are preferably electrical switches and thus, for example, incorporatetransistors. Switches 117, 118, 119 and 120 are entirely conventional.Switches 117 118, 1 19 and 120 are closed for alternate groups ofperiods. All the periods of one group are equal in time. All the periodsin the other group are likewise equal in time. Further, each period ofone group is equal to the common period of the other group. Switches117, 118, 119 and 120 are operated synchronously. That is, all-changeposition at the same time. However, switch 117 may be open or closedwhen any of the other switches 118, 119 and 120 are open or closed. Thesame is true ofswitch 120. Switch 118 may be open or closed regardlessof the open or closed states of switches 117 and 120. However, switch118 must be open when switch 119 is closed, and vice versa. The same istrue of switch 119.

Aresistor 121 connects the output of amplifier 115 to an amplifier 122.A resistor 123 is connected from the output of amplifier 122 to theinput thereof. The same is true of a resistor 124. Switches 1 18 and 119are connected on the output of amplifier 122 to ground. Switches 118 and119 are connected by a mutual junction 125. A capacitor 126 whichperforms the same function as capacitor 113 and is also relatively largeis connected from junction 125. A resistor 127 is connected fromcapacitor 126 to the input of an amplifier 128. A capacitor 129 isconnected from the output to the input of amplifier 128. The same istrue of switch 120.

The output of amplifier 128 is connected to the input of amplifier 130by a variable resistor R A variable resistor R is connected from asource of potential E, to the input of amplifier 130. Aresistor 131 isconnected from the output of amplifier 130 to the input thereof. Thesame is true of a capacitor 132. The output of amplifier 130 isconnected to the input of voltmeter 110. The output of amplifier 130 isa D.C. voltage directly proportional to where A and B are constants.From the previous equation f k f, it is also to be noted that by a merechange in constants, the output voltage of amplifier 130 is directlyproportional to where A, is also a constant.

As will be explained, the signal appearing at the left end of resistor127, as viewed in FIG. 11, will be a square wave having a maximumamplitude of +E and a minimum amplitude of E In FIG. 10, control 94 maybe entirely conventional except for transistor 106. Transistor 106, byitself, may be conventional, but not in the circuit combination or asused. Transistor 106 changes the resistance between junction 98 andground in accordance with the output of comparator 93. All the structureof control 94, except transistor 106, is simply a D.C. amplifier andfilter. Any conventional D.C. amplifier and filter may be substitutedtherefor.

Whether or not transistor 106 is considered in the circuit of control 94or in the circuit of filter 91, the major function of the filter 91 isto provide a passband with a tracking control through the connectionbetween transistor 106 and junction 98 via source 107.

Any conventional tracking filter and conventional control, therefore,may be substituted for filter 91 and control 94. Methods other thanphase comparison may also be employed to provide an input to thetracking filter frequency control.

In FIG. 11, resistor 14, capacitor 116, and amplifier form aconventional integrator. Any conventional integrator may be substitutedtherefor. The same is true of resistor 127, capacitor 129 and amplifier128.v

Resistor 121 with resistor 123, capacitor 124 and amplifier 122 form aconventional averaging circuit.

The output of amplifier 122 will be a substantially constant D.C.voltage directly proportional to the average value of the voltageappearing at the output of amplifier 115. The same is true of resistor Rresistor 131, capacitor 132 and amplifier 130. The fact that resistor Ris variable provides an adjustment feature in ac cordance with thedevice of the present invention. The same is true of resistor R That is,the output voltage of amplifier will be directly proportional to theaverage of the output voltage of amplifier 128, as modified by anyadjustments in resistors R and R,;.

For reference, the said integrators are indicated at 133 and 134. Theaveraging circuits are indicated at 135 and 136.

The input to integrator 133 is a square wave centered about zero volts.The output of integrator 133 would then be a series of triangularwavesbCircuit 122 shunts the output of amplifier 115 to its inputalternate triangles. Thus, the triangles are spaced between signalvalues very close to zero volts. Circuit 135 then averages thetriangles. Thus, the output of amplifier 122 is a substantially constantDC; voltage directly proportional to the peak amplitudes of thetriangles. Since the peak amplitudes are reached after an integration ofa square wave, the peak amplitudes are thus directly proportional toperiod, and inversely proportional to frequency. Hence, the output ofcircuit 135 is directly proportional to the reciprocal of frequency.Circuit 112 operates switches 118 and 119 to convert the output ofcircuit 135 again to a square wave having a maximum amplitude equal tothe output signal amplitude of circuit 135. This square wave is zeroadjusted through capacitor 126. so that, as stated previously, theconstant maximum is +E and theconstant minimum is E,. Integrator 134then produces triangular wave, as before, and circuit 112 operatingswitch 120 cancels out alternate triangles. Averaging circuit 136 thenproduces an output which is again directly proportional to the trianglepeaks. Since the triangle peaks are proportional to the product of E andthe triangle period, and E is proportional to the triangle period, theoutput of circuit 136 is proportional-to the period squared or inverselyproportional to the frequency squared. Thus, the output of circuit 136is proportional to f=( (1) where K is the force constant of the spring,and m is the total mass of the system.

Squaring both sides of (l and transposingf and m m K/f I (2) If m is themass of the container, and m, is the mass of c+ c (a) From (2) and (3) mm, K/f .both sides of (4),

. Subtracting m from By definition, mass is equal to the product ofdensity and volume. If the fluid has a density, d, and a volume, v, from(5 Combining (7),(8) and (9),

d=(A/f)+B 10 In accordance with the device of the present invention, itis striking that an output can be reduced from amplifier 130 that willbe directly proportional to d as defined in (10) within a very smallaccuracy tolerance of $0.01 percent over a wide range from about 0.08pounds per cubic foot to about 80.0 pounds per cubic foot. d thenbecomes the density of the liquid or gas under test, f is one of thefrequencies at which vane 20 resonates, preferably the lowest or firstresonant frequency. As stated previously, A and B are constants.

The unique character of the invention which causes the output ofamplifier 130 to follow d makes it possible to calibrate thedensitometer of FIG. 7 very easily, quickly and accurately.

The first step in calibration is to immerse probe 10 from entirely belowfitting 17 in a first fluid of known density d,,, and measure theresonant frequency f,,. The second step is to immerse probe 10 entirelybelow fitting 17 in a second fluid of known density d,,,' and measurethe resonant frequency, f,,, where d is not equal to d That is, thesecond fluid should not be the same fluid as the first fluid.

After the said calibration steps have been performed, the desiredconstants, A and B, may then be calculated from the followingsimultaneous equations (11) and (12), i.e., two equations and twounknowns.

In accordance with the foregoing, the resistance of variable resistor Rmay be changed by adjustment thereof until A is equal to the valuegivenby (14) and resistor R adjusted until B is equal to the value given by(19). Thus ...1= /f)+ l (20) where E is the DC. output voltage ofamplifier 130, and C is a constant of proportionality.

Notice the bracket terms have density units. MIL, and E has units of adifference of potential, e, where M mass, and L= length. Thus, Chasunits eLV/M. For example, C may bein units of volt cubic feet perpound.

Stated another way, E is directly proportional to the bracketed terms of(20), and the constant of proportionality, C, is simply determined bywhatever arbitrary voltage settings or amplifier gains that are selectedfor densitometer operation.

13 OPERATION In the operation of the densitometer shown in FIG. 7,ambient noise will cause detector 30 to pick up signals in a band offrequencies including the resilient frequency of the electromagneticoscillator. That is, signals will be amplified by amplifier 61,converted from a current to a voltage by converter 76, anddifferentiated by differentiator 82. The output of differentiator 82will thus be a series of alternate positive and negative pulses whichare converted into a square wave 83. Amplitude control 90 may be used toreduce the output of squarer 83 to a limiting value. The frequencylocation of the passband of tracking filter 91 will then be varied byfilter frequency control 94 to follow or pass the fundamental frequencyof the output of control 90 to power amplifier 92 with a minimumattenuation. The frequency location of the passband of tracking filter91 thus will be controlled through varying the resistance of resistor106, shown in FIG. 10. This will be done in accordance with thedifferentiation in phases of the output signals of control 90 and filter91 by phase comparator 93. Power amplifier 92 will then drive coil 24with a signal in phase with the resonant frequency signal output ofdetector 30. The vibration produced by coil 24 will the increase inamplitude until limited by amplitude control 90. At this time, theamplitude of the vibration will reach an approximately quiescent level.Should fluid be flowing in pipeline l9, and should the density of thefluid change, the frequency of the output signal of tracking filter 91will also change. Linearization circuit 109 will then produce a D.C.output voltage directly proportional to density. Indicator or voltmeter110 may then be read, when calibrated in density.

Note will be taken that parts of the probe are vibrated because thealternating signal applied to coil 24, as shown in FIG. 2, will placevarying amounts of radial compression on half cylinder 26, inner tube 21expanding and contracting axially between shank 40 and the abutment ofinner tube 21 at its lower end with its half cylinder 26.

It is an advantage of the device of the present invention thatdifferentiator amplifier 61 and converter 76 is used. For example,converter 76 may be located a great distance from probe 10. Converter76has a low resistance input and is thus a ground input device. Theaccuracy by which the resonant frequency signal output of detector 30 istransmitted to converter 76 is not affected by longitudinal transmissionline between probe 10 and converter 76. That is, the ground magnitude isaccurate regardless of the length of line. The voltage drop along theline, therefore, does not affect the accuracy when the resonantfrequency signal is transmitted to converter 76 from differentiatoramplifier 61. The rejection of external noise is also to be limited.

It is a feature of the invention that the interference fit of sleeve 22on half cylinders 26 and 27 is employed to improve efficiency andaccuracy. The use of crystal 30 also makes it possible to employ avibrating structure which is very small. The continuous compression fitof projection 39 between shank 43 and the upper side of half cylinder26, asshown in FIG. 2, also improves vibration efficiency. The operationof the leads from crystal 30 from the leads from coil24 is also asubstan tial advantage. The current through coil 24 thus cannot induce afeedback current in the crystal leads. Note that the crystal leads andthe coil leads are entirely separated and notice that they extend abovethe top of shield 53 in FIG. 2. As stated previously, shield 53 ismagnetic. So is fitting 50; so is washer 52; so is inner tube 21. Thecrystal leads are thus completely enclosed in a magnetic shield untilthey reach amplifier 61. Note, too, that shield 53 seals everythinginside thereof from coil leads 57 .and 58 until they reach the top ofshield 53. The output of amplifier 61, the induction from amplifier 61has raised the level of the input signal thereto to a substantial value.No substantial unwanted induction thus takes place between the coilleads and the output leads of amplifier leads 61 above the top of shield53 which, as stated previously, is made of magnetic material.

It is also an advantage of the invention that O-rings 46 and 47resiliently mount probe shank 11 through fitting 17. Substantiallyimproved efficiency and accuracy are to be achieved. The same is true ofthe resilient mount of electrical connector 14 through O-rings and 72.

One outstanding advantage of the present invention resides in the use ofdifferentiator 82 with tracking filter 91. Differentiator 82 acts, moreor less, as a high pass filter. If the amplitude of output signals ofdifferentiator 82 are plotted as a function of frequency, this amplitudewould be substantially a straight line of a predetermined positiveslope, amplitude being indicated positive vertically upward on theordinant, and frequency being indicated positive to the right. In spiteof the fact that differentiator 82 acts as a very good high pass filter,it provides a constant phase shift of input signals thereto. Trackingfilter 91 conveniently also provides such a phase shift at the frequencyat which signals are attenuated the least. Differentiator 82 andtracking filter 91 provide a phase shift in the same direction, i.e.,lead or lag. This means that the output of power amplifier 92 is analternating signal which may be adjusted in, phase, i.e., 180 or zero,simply by reversing the leads 57 and 58 from driver coil 24 connectedtherefrom. An in-phase drive is to be effected.

Another feature of the invention uses the double integration system toderive the function lf) B An outstanding advantage of the inventionresides in the use of switches 117, 118, 119 and 120 with circuit 112 toprevent any frequency modulation of the input to integrator 133 fromdestroying the accuracy of the output of circuit 136. q

The function of the double integration is easily understood when it isconsidered that the integral of E, a constant, with respect to X,results in EX, and the integral of EX results in EX/2. 1

Note will be taken that an outstanding feature of the invention residesin the use of probe 10. Probe 10 may be used with any pipe size and hassubstantial sensitivity and accuracy. No pressure differentiators orchanges are involved. The densitometer of the present invention may havean accuracy tolerance of 10.01 percent over a range of, for example,0.08 pounds per cubic foot to 80.0 pounds per cubic foot. Vane 20 issmall and compact and does not disturb the flow of pipeline 19. The sameis true of other structures surrounding vane 20.

inner tube 21, and boss 36 and sleeve 22. That is, inner tube 21 is notbonded or otherwise similarly fixed to boss 36 or sleeve 22, but isonly, more or less, slidabie therethrough. What is meant by not fixed inthe immediately preceding description is that, further, there is nopress fit. Similarly, the inner end of inner tube 21 is not fixed tohalf cylinder 26.

Note will be taken that one feature of the invention may be used withoutany of the other features. Moreover, any one feature may be used withany one or more or all of the other features. The invention, therefore,is not to be limited to any one feature.

' What is claimed is:

1. The method of calibrating a vibration densitometer, said method ofcomprising the steps of: measuring a resonant frequency, f},, of thedensitometer in a first fluid of known density, (1,; measuring the sameresonant frequency, 1],, of the densitometer in a second fluid of knowndensity, d,,; causing the densitometer to produce an output directlyproportional to the expres- SlOn (Alf) B where,

f is the densitometer resonant frequency, and

A and B are constants; v and adjusting the densitometer until A becomesequal to the expression and B becomes equal to the expression l I t

1. The method of calibrating a vibration densitometer, said method ofcomprising the steps of: measuring a resonant frequency, fa, of thedensitometer in a first fluid of known density, da; measuring the sameresonant frequency, fb, of the densitometer in a second fluid of knowndensity, db; causing the densitometer to produce an output directlyproportional to the expression (A/f2) + B where, f is the densitometerresonant frequency, and A and B are constants; and adjusting thedensitometer until A becomes equal to the expression (fa2 fb2 (da -db))/(fb2 - fa2) and B becomes equal to the expression (db fb2 - dafa2)/(fb2 fa2)